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From "Kaktu Chakarabati (JIRA)" <>
Subject [jira] Created: (SOLR-1149) Make QParserPlugin and related classes extendible
Date Thu, 07 May 2009 07:55:33 GMT
Make QParserPlugin and related classes extendible

                 Key: SOLR-1149
             Project: Solr
          Issue Type: Improvement
          Components: search
            Reporter: Kaktu Chakarabati
             Fix For: 1.4

In a recent attempt to create a QParserPlugin which extends DisMaxQParser/FunctionQParser
it became apparent that in the current state of these classes, it is not straight forward
and in fact impossible to seriously build
upon the existing code. 
To this end, I've refactored some of the involved classes which enabled me to reuse existing
logic to great results.
I thought I will share these changes and comment on their nature in the hope these will make
sense to other solr developers/users, and
at the very least cultivate a fruitful discussion about this particular area of the solr codebase.

The relevant changes are as follows:

* Renamed DismaxQParser class to DisMaxQParser ( in accordance with the apparent naming convention,
e.g DisMaxQParserPlugin )

* Moved DisMaxQParser to its own .java file, making it a public class rather than its previous
package-private visibility. This makes
  it possible for users to build upon its logic, which is considerable, and to my mind is
a good place to start alot of custom
  QParser implementations.

* Changed access modifiers for the QParser abstract base class to protected (were package-private).
Again as above, it makes this
  object usable by user-defined classes that wish to define custom QParser classes. More generally,
and on the philosophy-of-code 
  side of things, it seems misleading to define some class members as having the default access
modifier (package-private) and then
  letting other package-scope derived classes use these while not explicitly allowing user-defined
derived classes to make use of these members.

  In specific i'm thinking of how DisMaxQParser makes use of these members: **not because
it is derived from QParser, but because it
  simply resides in the same namespace**

* Changed access modifier for the QueryParsing.StrParser inner class and its constructors
to public. Again as in above, same issue
  of having same-package classes enjoy the benefit of being in the same namespace (FunctionQParser.parse()
uses it like so), 
  while user defined classes cannot. Particulary in this case it is pretty bad since this
class advertises itself as a collection of utilities
  for query parsing in general - great resource, should probably even live elsewhere (common.utils?)

* Changed Function.FunctionWeight inner class data member modifiers to protected (were default
- package-private). This allowed me
  to inherit from FunctionQuery as well as make use of its original FunctionWeight inner class
while overriding some of the latter's
  methods. This is in the same spirit of the changes above. Please also note this follows
the common Query/Weight implementation pattern
  in the lucene codebase, see for example the BooleanQuery/BooleanWeight code.

All in all these are relatively minor changes which unlock a great deal of functionality to
3rd party developers, which i think is
ultimately a big part of what solr is all about - extendability. It is also perhaps a cue
for a more serious refactoring of the
QParserPlugin hierarchy, although i will leave such bold exclamations to another occasion.

Attached is a patch file, having passed the usual coding-style/unit testing cycle.


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